In recent years, as a technique for enabling large-capacity data communication such as images, attention has been directed toward MIMO (Multi-Input/Multi-Output) communication. In MIMO communication, the transmitting side transmits different items of transmission data (substreams) from a plurality of antennas, and a receiving side separates the original transmission data from a plurality of items of transmission data mixed over the channel using estimation values (for example, see Patent Document 1).
Actually, in MIMO communication, signals transmitted from a transmitting apparatus are received using the same number of antennas as the antennas of the transmitting apparatus or more, and, based on a pilot signal inserted in a signal received in each of the antennas, channel characteristics between the antennas are estimated. The estimated channel characteristics H are expressed by a 2×2 matrix when the number of transmitting side antennas is two and the number of receiving antennas is two. In MIMO communication, a transmission signal (substream) transmitted from each transmitting antenna is obtained based on the inverse matrix of the obtained channel characteristics H and a received signal obtained in each receiving antenna.
Further, automatic repeat request (ARQ) is generally performed whereby, when the bit error rate on the receiving side does not meet a predetermined value, the receiving side transmits a retransmission request signal to the transmitting side and, in response to the request, the transmitting side transmits the same transmission data again. Particularly, in packet transmission, data transmission without errors needs to be guaranteed, and this makes error control by ARQ indispensable. In addition, in packet transmission, although adaptive modulation and error correction are applied such that throughput increases by selecting an optimal modulation scheme and coding scheme in accordance with the state of the channel, packet errors are inevitable due to measurement error, control delay and the like, so that hybrid ARQ (hereinafter referred to as HARQ) is adopted incorporating FEC (Forward Error Correction) functions.
Thus, MIMO communication is performed using a plurality of antennas during data transmission, so that large-capacity data can be communicated, and, when received data is erroneous on the receiving side, data is retransmitted, and the receiving side combines the received data in the initial transmission and in retransmission by HARQ. By this means, increased throughput of wireless communication system is expected. Non-patent Document 1 also suggests applying HARQ to MIMO communication.
Patent Document 1: Japanese Patent Application Laid-Open No. 2002-44051
Non-patent Document 1: 3GPP TSG-RAN Working Group 1, Collection, R1-010879, “Increasing MIMO throughput with per-antenna rate control”, Lucent Technologies